1. Field of the Invention
The present invention relates to a torque sensor for detecting torque applied to a rotating shaft based on measurement of changes in magnetic flux density or magnetic field intensity.
2. Description of the Related Art
The present inventors have filed a prior patent application relating to a torque sensor (Japanese Patent Application No. 2001-316788, U.S. Patent Application Publication No. 2002-189371). This torque sensor generally includes a torsion bar connecting an input shaft and an output shaft, a multi-pole magnet fixed to one end of this torsion bar, a pair of multi-pole yokes fixed to the other end of the torsion bar and disposed inside a magnetic field generated by the multi-pole magnet, a pair of magnetic flux collecting rings for introducing magnetic flux from the multi-pole yokes, and a magnetic sensor for detecting magnetic field intensity via the rings.
The pair of magnetic flux collecting rings are each provided with portions that have the function of collecting magnetic flux directed from the multi-pole yokes, and these portions are arranged opposite to each other. The magnetic sensor is interposed between these mutually opposing magnetic flux collecting portions of the rings. When the torsion bar is twisted, magnetic flux is generated between the pair of multi-pole yokes due to a change in the relative positional relationship between the multi-pole magnet and multi-pole yokes in the circumferential direction. The magnetic flux is collected by the pair of rings, and the magnetic sensor detects the density of the magnetic flux.
It is crucial, in this torque sensor, to utilize the magnetic flux collected by the rings as much as possible. However, because the magnetic sensor is interposed between the magnetic flux collecting portions of the rings, there exists a problem of magnetic flux leakage through the inevitable air gaps between the magnetic sensor and magnetic flux collecting portions. This leakage causes a decrease in signal output from the sensor.
Another problem is that, since the air gaps in each and every product cannot possibly be made constant because of slight variations resulting from assembling tolerances, the signal output from the sensors tends to be slightly different.